It is known that satellite radio telephones have been developed and deployed throughout the world, especially where topographical conditions preclude use of conventional mobile radio telephones or fixed wire telephones, for example in rugged terrain areas or sparsely populated areas. These systems typically are understood to provide both voice and data communications so that they may include terminals such as PCS terminals.
Prior art satellite radio telephone systems may be broadly classified into two types: mobile satellite radio telephone systems and fixed satellite radio telephone systems. In each of these systems, one or more satellites are used to communicate with radio telephones, the satellites being either orbiting satellites or geostationary satellites. Typically, a mobile satellite radio telephone system is designed to communicate with a plurality of mobile radio telephones of similar size as conventional cellular radio telephones, whereas a fixed satellite radio telephone system is designed to communicate with a plurality of fixed or non-mobile radio telephones using large, permanent or semi-permanent fixed antennas which may be mounted on buildings or homes. The satellite signals to/from the fixed radio telephones are usually further distributed to a large number of users via a terrestrial, wired network.
Consequently, it has been found that the capacity of fixed satellite radio telephone systems (as measured by total number of users served) is much larger than that associated with mobile satellite radio telephone systems. In particular, a significantly lower number of users is served than in a fixed satellite system.
Generally, the per-satellite capacity of a satellite radio telephone is limited by the amount of satellite power that is expended per communication circuit in order to establish and maintain communications with a radio telephone. In addition, limiting factors such as available frequency spectrum and the typically poor frequency reuse of satellite radio telephone systems impact the per-satellite capacity of these phones.
Consequently, mobile satellite radio telephone systems have been found to generally possess much lower capacity than fixed satellite radio telephone systems. Both regional and global mobile satellite systems are found to be quite limited in capacity. Regional mobile satellite systems involving geostationary satellites have, for example, a capacity per satellite of about 10,000 simultaneous voice circuits. Global mobile satellite systems involving both medium earth orbiting satellites, or MEOs, or low earth orbiting satellites, or LEOs, generally have even lower capacity per satellite, having on the order of 3,000-4,000 simultaneous voice circuits.
It is understood that the per-satellite capacity of mobile satellite radio telephone systems is the amount of power that is expended per communication by the satellite payload in order to establish and maintain communications with the small hand-held mobile phones. The practical limitations involved in forming a very large number of spot beams from the satellite often limits frequency reuse for mobile satellite radio telephone systems, resulting in generally low capacities of the systems.
In contrast, fixed satellite radio telephone systems generally have higher capacity than mobile systems, since they employ fixed user terminals which can use relatively large end user antennas. The satellite power required per equivalent communication is found to be lower for a fixed system than for a mobile system. In addition, frequency allocations of the fixed systems are found to be more liberal than those of mobile systems due to the fact that fixed systems generally operate at higher frequencies, such as C-band or above, where the frequency spectrum is not as crowded, and user terminals are fixed and use highly directional antennas so that angular separation between satellites can be relied upon for frequency reuse.
Prior art methods have been developed to increase the relatively limited capacity of mobile satellite radio telephone systems experiencing capacity bottlenecks or hot spots which are developed in congested areas of the mobile satellite radio telephone system where the mobile satellite radio telephone system (MSS) does not have enough capacity to accommodate all users. It has been found that it is difficult to increase the capacity of the MSS in these congested areas.
Consequently, methods and systems to increase the capacity of a MSS include allowing an MSS to use some of the capacity of a fixed satellite system in areas of congestion, such as disclosed in U.S. Pat. No. 6,052,586.
In addition to improving cellular satellite communication systems and methods to provide wireless communications employing at least one space based component, such as one or more satellites that are configured to wirelessly communicate with a plurality of radio telephones or other types of cellular terminals, hybrids of satellite and terrestrial systems have been developed and used wherein terrestrial networks enhance cellular satellite communications system availability, efficiency and/or economic viability by terrestrially reusing at least some of the frequency bands allocated to cellular satellite communication systems.
Cellular satellite communication systems experience difficulty in reliably serving densely populated areas where the satellite signal may be blocked by high rise structures or may not penetrate into buildings. In such cases, the satellite spectrum may be underutilized or unutilized in such areas. It is found that the use of terrestrial retransmission can reduce or eliminate this problem. Thus, the capacity of the overall system can be increased significantly by the introduction of terrestrial retransmission since terrestrial frequency reuse can be much denser than that of a satellite-only system. It is further found that capacity can be enhanced where it may be mostly needed, for example, densely populated urban/industrial/commercial areas so that the overall system can become much more economically viable as it may be able to serve a much larger subscriber base.
One example in the prior art of terrestrial reuse of satellite frequencies is described in U.S. Pat. No. 5,937,332 entitled “Satellite Telecommunications Repeaters and Retransmission Methods”. As is generally described therein, satellite communication repeaters are provided which receive, amplify and locally retransmit the downlink signal received from a satellite, thereby increasing the effect of downlink margin in the vicinity of the satellite telecommunications repeaters and allowing an increase in the penetration of uplink and downlink signals into buildings, foliage, transportation vehicles and other objects which can reduce link and margin.
Methods and systems in the prior art allow a satellite radio telephone frequency to be reused terrestrially within the same satellite cell while allowing intrasystem interference to be reduced. These systems include a space-based component such as a satellite that is configured to receive wireless communications from a first radio telephone in a satellite footprint comprising one or more cells over a satellite radio telephone frequency band. An ancillary terrestrial network is also provided that comprises one or more ancillary terrestrial components configured to receive wireless communications from a second radio telephone in the satellite footprint over the satellite radio telephone frequency band. The wireless communications from the second radio telephone are also received by the space based component in the satellite footprint over the satellite radio telephone frequency band as interference, along with the wireless communications that are received from the first radio telephone in the satellite footprint over the satellite radio telephone frequency band. In such cases, an interference reducer is employed that is responsive to the space based component and to the ancillary terrestrial network and that is configured to reduce the interference from the wireless communications that are received by the space based component from the first radio telephone in the satellite footprint over the satellite radio telephone frequency band using the wireless communications that are received by the ancillary terrestrial network from the second radio telephone in the satellite footprint over the satellite radio telephone frequency band.
Other wireless communications systems including a satellite gateway coupled to a communications network and operative to communicate with a communications satellite include a terrestrial terminal interface subsystem operative to communicate with a satellite gateway via the communications satellite using a first radio interface and to communicate with wireless terminals over a geographic area using a second radio interface, for example, as is discussed in U.S. Pat. No. 6,856,787.
Other cellular systems include a space based system having a first set of cells and a ground based system having a second set of cells. In such systems the space and ground systems can optionally function substantially autonomously with each using spectrum from at least one predetermined frequency band, for example, as is discussed in U.S. Pat. No. 6,859,652.
Prior art mobile satellite systems employing radio telephones or MSS are known in the art; for example, in U.S. Pat. No. 5,303,286 assigned to Globalstar®, a satellite communication system has at least one, but usually a plurality, of orbiting satellites over a terrestrial satellite service area, a satellite control center and a plurality of terrestrial communication links, wherein call setup is controlled by processors and databases onboard the orbiting satellites, and wherein only after the satellite link for the communication channels is completed, does control and switching rely on ground-based systems such that the orbiting satellites are integrated into a ground based telephone network and tariff structure.
In U.S. Pat. No. 5,715,297 assigned to Globalstar®, there is disclosed a radio communication system capable of servicing a roaming user or the like outside the range of terrestrial relay stations which includes a packet switched network and database of roaming users and a satellite communications system having at least one, but usually a plurality, of orbiting satellites over a terrestrial satellite service area, a satellite control center and a plurality of terrestrial communication links, wherein call setup is controlled by processors and databases onboard the orbiting satellites, and wherein only after the satellite link for the communication channels is completed, does control and switching rely on ground based equipment such that the orbiting satellites are integrated to a ground based telephone network and tariff structures. Similar systems and improvements thereto, as found in the U.S. Pat. Nos. 5,303,286 and 5,715,297 patents, include those discussed in U.S. Pat. No. 5,903,837 and U.S. Pat. No. 6,072,768. In addition, still other systems are discussed in U.S. Pat. Nos. 5,905,943 and 6,879,829.
Various other systems have been proposed as depicted in the FCC filing for “Authority to Launch and Operate a Satellite System to Provide Mobile Satellite Services in the 2 GHz Bands” dated Nov. 3, 2000, relating to the Globalstar® system, which is hereby incorporated by reference; the FCC filing in the matter of Mobile Satellite Ventures Subsidiary, LLC for “Minor Amendment of Application to Launch and Operate a Replacement L Band Mobile Service Satellite at 1010 West” dated Nov. 18, 2003; and the FCC filing by Thuraya which depicts a one GEO satellite system to provide a satellite telephone service; and the Iridium system produced by Motorola generally described in U.S. Pat. Nos. 5,918,176 and 5,490,087, in addition to the above recited Globalstar® systems.
In view of the above discussion, there continues to be a demonstrated need for systems and methods for terrestrial reuse of cellular satellite frequencies that can allow improved reliability, capacity, cost effectiveness and/or esthetic appeal for cellular satellite radio telephone systems, methods and/or satellite radio telephones. This is especially significant in view of the enormous costs associated with providing the satellites and launching them, in addition to monitoring them in orbit to efficiently and effectively provide satellite telephone services.
More recently satellite radio telephone systems have been provided employing multiple air interfaces and multiple frequency bands operating on the same satellite system having an ancillary terrestrial component (ATC). These prior art systems, such as that disclosed in U.S. Pat. No. 7,113,778 issued to Karabinis, are directed to mobile satellite systems (MSS) operators which have sufficient spectrum to provide an ATC component with the necessary frequency reuse. These and other systems, such as is disclosed in U.S. Pat. No. 7,113,778, are ineffective to provide a satellite radio telephone system having at least two operators in the MSS/ATC mode where each operator is found to have insufficient spectrum for efficient reuse in the ATC mode. Currently, systems are being developed to address this problem and provide an effective satellite radio telephone system employing multiple satellite systems with different air interfaces for the MSS component. These systems are to be contrasted with those provided in the prior art as in, for example, the system disclosed in U.S. Pat. No. 7,113,778 which is directed to one satellite system having multiple air interfaces. There is therefore seen to be a demonstrated need to provide systems devoid of the above noted prior art deficiencies.